(1) Field of the Invention
This invention relates to a substrate treating apparatus for treating, with a treating solution, substrates such as semiconductor wafers, glass substrates for liquid crystal displays, glass substrates for photomasks and the like (hereinafter called simply “substrates”).
(2) Description of the Related Art
Known apparatus of this type include, for example, an apparatus for etching a silicon nitride film on substrate surfaces with a high-temperature phosphoric acid solution (H3PO4) (see Japanese Unexamined Patent Publication No. 9-181041 (1997), page 3 and FIG. 1, for example).
The concentration of the solution may be controlled simultaneously with and independently of its temperature (see Japanese Unexamined Patent Publication No. 11-154665 (1999), paragraphs [0015] to [0017] and FIG. 1, for example).
A known apparatus will be described hereinafter with reference to FIG. 1. This apparatus includes a treating tank 101 for storing a phosphoric acid solution, a collecting tank 102 for collecting the phosphoric acid solution overflowing the treating tank 101, a circulating system 103 for returning the phosphoric acid solution collected in the collecting tank 102 to the treating tank 101, a heater 104 for constantly heating the phosphoric acid solution in the treating tank 101, a phosphoric acid supply device 105, and a supplementing device 106 for supplementing the treating tank 101 with deionized water for controlling the temperature of the phosphoric acid solution.
In order to maintain a high etching rate for silicon oxide film, the phosphoric acid solution adjusted to a predetermined concentration is maintained at a high temperature (120 to 170° C.) just below a boiling point corresponding to that concentration. When the phosphoric acid solution is above the predetermined temperature, the solution is cooled by supplying deionized water from the supplementing device 106 into the treating tank 101. Conversely, when the phosphoric acid solution is below the predetermined temperature, the solution is heated by the heater 104 without supplementing deionized water.
Wafers W to undergo etching treatment are placed in the treating tank 101 as arranged in vertical posture and equidistantly on a horizontal guide 107a of an approximately L-shaped holding arm 107. The treating tank 101 has an openable cover 108 for covering an upper opening thereof through which the group of wafers W held by the holding arm 107 is loaded into and unloaded from the tank 101. The cover 108 is closed when the holding arm 107 is outside the treating tank 101. The cover 108 is opened for allowing the group of wafers W to be loaded into the treating tank 101. After the group of wafers W is loaded into the treating tank 101, the cover 108 is closed again and etching treatment is carried out in the state shown in FIG. 1. In this state, a suspension arm 107b of the holding arm 107 extends out of the tank 101 through an opening 108a formed in the cover 108.
The conventional apparatus described above has the following drawback.
When the group of wafers W in a “lot-out” state outside the treating tank 101 is moved to a “lot-in” state inside the treating tank 101, the concentration of the phosphoric acid solution in the treating tank 101 undergoes a marked change. Such a change in the concentration of the phosphoric acid solution lowers the quality of etching treatment. Further, the lot-in state may entail a sudden boiling (bumping) of the phosphoric acid solution. The bumping of the phosphoric acid solution results in violent movements thereof, which vibrate and adversely influence the wafers W.
The method of controlling concentration and temperature simultaneously with and independently of each other can easily control the concentration with high precision when there is no disturbance and the temperature is stable. However, since the temperature is seldom maintained stable, this method has the problem of taking a long time for the concentration to reach a set value.
Such a problem will be described by referring to a specific example of variations in the temperature and concentration (specific gravity) shown in FIG. 2.
In this example, as shown in FIG. 2A, heating is started at time t=0 to increase temperature from tm0 toward a set temperature tm1. In parallel therewith, as shown in FIG. 2B, concentration is controlled from cn0 toward a set concentration cn1, and deionized water is supplied for this purpose. Then, the concentration is lowered by the deionized water supply and heating. While heating is continued to increase the temperature toward the set temperature tm1, the concentration falls below the set concentration cn1 once before the temperature reaches the set temperature tm1. The concentration reaches the set concentration cn1 after time t2 when the temperature reaches the set temperature tm1. That is, by controlling the temperature and concentration simultaneously, the concentration undergoes variations under the influence of the temperature. The variations in the concentration are greatly influenced also by the deionized water supplied for concentration control. This results in the problem noted above.
In the above apparatus, the supplementing device or switch valve 106 can take either an open position or a closed position.
Thus, the time of the switch valve 106 being kept open is adjusted to control an amount of deionized water supplemented. It is difficult for the conventional apparatus to supply an accurately adjusted amount of deionized water.
In the above conventional example, in order to maintain a high etching rate, the phosphoric acid solution adjusted to a predetermined concentration is maintained at a high temperature just below the boiling point corresponding to that concentration. The problem noted above occurs also with a different type of substrate treating apparatus in which the switch valve 106 is operated to supplement deionized water or other diluent for a chemical solution to the treating tank 101.